How to get a feature requirement tag in the documentation generated by `cargo doc`?

If you look at the Tokio docs on docs.rs there's a blue tag indicating that a feature must be activated in order to access this API:

I would like to enable this for my crate as well, how can this be done?

The bad news is: It's a nightly-only feature for now.

The good news is: docs.rs uses nightly by default.

To get this to work all you need is to enable the doc_cfg feature and apply #doc(cfg) to the item being documented

#![feature(doc_cfg)]

#[doc(cfg(feature = "macros"))]
pub fn test() {}

Because this is a nightly-only feature, you probably don't want to enable it all the time. tokio defines the following in its Cargo.toml to only enable this feature on docs.rs:

# docs.rs-specific configuration
[package.metadata.docs.rs]
# document all features
all-features = true
# defines the configuration attribute `docsrs`
rustdoc-args = ["--cfg", "docsrs"]

and then they use

// only enables the `doc_cfg` feature when
// the `docsrs` configuration attribute is defined
#![cfg_attr(docsrs, feature(doc_cfg))]

#[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
pub fn test() {}